Suppose you see a round red ball. This appearance of redness and roundness - what is it?

David Rosenthal, a philosopher of mind, lays out two ways of approaching "mental qualities". There is the consciousness-based approach, which posits that only consciousness gives us access to perceptual "qualia" like color and smell. And there is the perceptual-role approach: the view that we know about mental qualities through their role in perceptual discrimination. Rosenthal argues for the latter and develops a "quality space theory" to describe what mental qualities are in these terms. Along the way, he discusses subliminal perception and priming effects, the "undetectable inversion" hypothesis (basically this idea), and why he thinks there is no "hard problem of consciousness" at all.